ISDN is capable of transferring data messages transparently between ISDN telephones. ISDN operates according to the call control methods and procedures defined by Bellcore in the National ISDN Technical Requirements. The Bellcore requirements specify information elements such as user-to-user information, calling sub-address information, and lower layer compatibility. ISDN also has the capability of sending X.25 data on the network. These information elements do not exist in any prior art analog service. ISDN basic rate service operates on a 2B+D configuration, that is, two 64 kilobyte per second B or bearer channels and one 16 kilobyte per second D or delta channel.
It has been proposed to use ISDN information elements to pass a calling party number received at a first ISDN telephone to a second ISDN telephone prior to transferring the call to the second telephone. See, for example, the U.S. Pat. No. to Kakwashima 5,422,942. One problem with this prior art call transfer method is that communication between the calling party and the second ISDN telephone is completed through the first ISDN telephone, occupying at least one channel of the first ISDN telephone until completion of the call.